This invention relates to improvements in a ringing choke type DC/DC converter for controlling in response to variations in a load, an input voltage and an environmental temperature.
FIG. 1 is a circuit illustrating a prior-art ringing choke type DC/DC converter (hereinbelow referred to as an "R.C.C") disclosed, for example, in FIG. 6 on page 214 of TRANSISTOR TECHNIQUE, issued in September, 1977 (by CQ Publication Co., Japan). In the drawing, numeral 1 denotes a d.c. power source and numeral 2 a transformer having a primary winding 3, a feedback winding 6, and a secondary winding 2. The starting end of the primary winding 3 is connected to the positive electrode of the d.c. power source 1, and the finishing end of the primary winding 3 is connected to the collector of a transistor 4. The emitter of the transistor 4 is connected to the negative electrode of the d.c. power source 1. Numeral 5 denotes a starting resistor connected between the positive electrode of the d.c. power source 1 and the base of the transistor 4. The starting end of the feedback winding 6 is connected through a series circuit including a resistor 7 and a capacitor 8 to the base of the transistor 4, and the finishing end of the feedback winding 6 is connected to the emitter of the transistor 4. Numeral 9 denotes a diode having the cathode side connected to the starting end of the feedback winding 6, and the anode side connected to the negative electrode of a capacitor 10. The positive electrode of the capacitor 10 is connected to the finishing end of the feedback winding 6. Numeral 11 denotes a Zener diode having the cathode side connected to the base of the transistor 4, and the anode side connected to the negative electrode of the capacitor 10. Numeral 13 denotes a diode having the anode side connected to the finishing end of the secondary winding 12, and the cathode side connected to the positive electrode of a capacitor 14. The negative electrode of the capacitor 14 is connected to the starting end of the secondary winding 12. Numeral 15 denotes a load connected to both ends of the capacitor 14.
The self-excited oscillating operation of the R.C.C will be described with reference to FIG. 2 which illustrates the waveforms of respective points as a function of operating time. When a power switch, not shown, is turned on, a voltage Vin from the d.c. power source 1 is applied across the starting resistor 5 and the transistor 4 to provide a base current i.sub.B flowing through the base of the transistor 4. Thus, the transistor 4 is turned on to allow a current, hereinafter referred to as the collector current i.sub.C, to flow through the primary winding 3. A voltage generated in the feedback winding 6 by the magnetomotive force of the primary winding 3 due to the collector current i.sub.C is differentiated by the resistor 7 and the capacitor 8 to provide base current with a differentiated waveform so as to quickly saturate the transistor 4. On the other hand, since this same voltage is reversely applied to the diode 9 which is used for rectifying a load output of the transistor 4 in this saturated state, no current flows therethrough. Hence, the collector current i.sub.C becomes i.sub.C =h.sub.fe .multidot.i.sub.B, where h.sub.fe denotes the current amplification factor of the transistor 4. At this time, the magnetic flux in the core of the transformer 2 becomes constant, and no voltage is generated in the respective windings. As a result, the transistor 4 is rapidly shifted to an off state. When the transistor 4 becomes the off state, a vibrating voltage of reverse polarity to the on state of the transistor 4 is generated in the windings by the magnetic energy stored in the transformer 2 during the flow of collector current i.sub.C. Therefore, a voltage of the direction for turning ON the diode 13 is output to the secondary winding 12 at this time to charge the capacitor 14 and to supply a power to the load 15. Here, a voltage of reverse direction between the base and the emitter of the transistor 4 is similarly generated in the feedback winding 6 to charge the capacitor 8 through the resistor 7 so that the electrode side connected to the base of the transistor 4 becomes positive. Here, since a variation in the magnetic flux in the core of the transformer 2 becomes constant when the magnetic energy stored during the on period is discharged to all the loads of the windings, the voltages of the windings of the transformer 2 tend to be cancelled. Here, the base current i.sub.B flows as a forward base current from the capacitor 8 to the transistor 4 in response to the variation in the voltage, and the transistor 4 again becomes ON state. Thus, the transistor 4 alternatively repeats the ON state and the OFF state to repeat the switching of the transistor 4, thereby continuing a self-excited oscillation.
Then, a mechanism for controlling the voltage in FIG. 1 will be described. As described above, the feedback winding 6 generates a voltage in the direction for turning ON the diode 9 during the OFF period of the transistor 4 to charge the capacitor 10. Thus, it is considered that the voltage across the capacitor 10 is substantially proportional to an output voltage Vo and an input voltage Vin. Therefore, the sum of the charging voltage of the capacitor 10 and the induced voltage of the feedback winding 6 is applied to the Zener diode 11 when the transistor 4 is next turned on, and a Zener current proportional to the difference between both the voltages flows therethrough. Thus, a part of the current supplied from the feedback winding 6 to the base of the transistor 4 is bypassed as the Zener current to control the base current i.sub.B of the transistor 4, thereby controlling the ON duration of the transistor 4 to act so that the output voltage Vo becomes stable irrespective of the input voltage Vin and the load 15. The operating principle of controlling the voltage described above will be further described in more detail with an equivalent circuit shown in FIG. 3.
Since FIG. 3 is shown for the convenience of describing the voltage control, the capacitor 8 which does not relate directly to the voltage control is omitted. The base emitter junction of the transistor is represented by a linear type in the equivalent circuit in FIG. 3 if the base current i.sub.B flows forwardly in the transistor 4, and the voltage V.sub.BE between the base and the emitter is to be represented by the following equation (1). EQU V.sub.BE =r.sub.B i.sub.B +V.sub.B ( 1)
where r.sub.B denotes the operating resistance of the transistor 4, and V.sub.B denotes a junction barrier voltage. Similarly, if the voltage applied between the anode and the cathode of the Zener diode 11 becomes as high as the Zener voltage, the Zener current i.sub.Z flows, and the Zener diode voltage V.sub.ZD is to be represented by the following equation (2) in a linear type. EQU V.sub.ZD =r.sub.Z i.sub.Z +V.sub.Z ( 2)
where r.sub.Z denotes the operating resistance of the Zener diode 11, and V.sub.Z denotes a Zener voltage. Since the equivalent circuit of FIG. 3 is shown at a timing that the transistor 4 is turned ON, a feedback voltage of V.sub.f is generated in the feedback winding 6 in the direction as shown in FIG. 3. Further, the capacitor 10 is charged to a voltage produced by subtracting the voltage V.sub.f of the feedback winding 6 by the ON voltage of the diode 9 during the OFF period of the transistor 4 and hence a voltage V.sub.Cf. If the ON voltage of the diode 9 is extremely low so as to be ignored, it is said that the charging voltage V.sub.Cf is substantially equal to the voltage V.sub.f of the feedback winding 6 during the OFF period of the transistor 4. The capacitor 10 has a capacitive component C.sub.f and an impedance component r.sub.Cf . A current flowing to the resistor 7 is represented by i.sub.f. Here, assume that the feedback current i.sub.f is branched to the base current i.sub.B and the Zener current i.sub.Z to be the state of the following equation (3), EQU i.sub.B =i.sub.f -i.sub.Z ( 3)
the following equation (4) is satisfied. ##EQU1## In the equation (4), ##EQU2## denotes the ripple voltage of the capacitor 10. If r.sub.B i.sub.B is smaller than the other terms to be ignored and C.sub.f is sufficiently large to make the term ##EQU3## negligible, the equation (4) can be expressed by the following equation (5). EQU V.sub.B =(r.sub.Z +r.sub.Cf)i.sub.Z +V.sub.Z -V.sub.Cf ( 5)
Therefore, the i.sub.Z is represented by the following equation (6) from the equation (5). ##EQU4## If the equation (5) is transformed in term of the V.sub.Z, the following equation (7) is attained. EQU V.sub.Z =V.sub.B +V.sub.Cf -(r.sub.Z +r.sub.Cf)i.sub.Z ( 7)
Here, when the Zener diode 11 is not turned on, i.sub.Z =0. Therefore, the equation (7) can be represented by the following equation (8). EQU V.sub.ZD =V.sub.B +V.sub.Cf ( 8)
If the input voltage Vin is raised or the load 15 is lightened so that the charging voltage V.sub.Cf increases and the V.sub.ZD in the equation (8) become equal to or greater than V.sub.Z as below, EQU V.sub.ZD .gtoreq.V.sub.Z ( 9)
the Zener current i.sub.Z represented by the equation (6) flows. Here, if the input voltage Vin is raised or the load 15 is lightened, the feedback voltage V.sub.f rises and the feedback current i.sub.f also increases, but since the charging voltage V.sub.Cf also increases so that the Zener current i.sub.Z increases, the base current i.sub.B flowing to the transistor 4 is limited by the equation (3) to shorten the ON width, thereby controlling so that the charging voltage V.sub.Cf becomes a target value V.sub.Z.
The above description is the detailed mechanism of the voltage control.
Since the prior-art R.C.C is constructed as described above, if the input voltage Vin is raised, the load 15 is lightened or the environmental temperature is lowered, there is a problem that the oscillation of the transistor 4 becomes intermittent to cause the output voltage Vo to become unstable. Thus, in order to stably operate the R.C.C., it is necessary to limit the input voltage Vin, to apply a dummy load to the load 15, to limit the environmental temperature or to increase the value of the output capacitor 14 so as to prevent intermittent oscillation from occurring and to stabilize the output even if the oscillation becomes intermittent. Additional problems will be further described in detail. The indirect control of the output voltage Vo on the basis of the base current i.sub.B by controlling the Zener current i.sub.Z according to the equation (6) if the input voltage Vin is raised or the load 15 is lightened to cause the Zener current i.sub.Z to increase was described above. However, since the term (r.sub.Z +r.sub.Cf)i.sub.Z of equation (7) increases as the Zener current i.sub.Z increases, the Zener voltage V.sub.Z does not satisfy the equation (9) and the Zener diode 11 remains off. Thus, the base current i.sub.B becomes extremely larger than the target value to cause the excessive current to flow, so that the ON period of the transistor 4 becomes extremely longer than the target value, with the result that the output voltage Vo largely increases. Therefore, since V.sub.f increases, V.sub.Cf also increases, the thereby decreasing resultant voltage so that the state during which the base current i.sub.B does not flow is continued until the V.sub.Z of the equation (7) satisfies the equation (9). As a result, the transistor 4 is caused to intermittently oscillate, and the stability of the output voltage Vo is largely lost due to the intermittent oscillation.
If the environmental temperature varies, the values of r.sub.Cf, r.sub.Z and i.sub.B alters, and the intermittent oscillation tends to feasibly occur.
Furthermore, when the capacity C.sub.f of the capacitor 10 is large, the physical size of the capacitor 10 increases, thus increasing circuitry space and manufacturing cost.